This invention relates to synchronous rectifiers comprising a field-effect transistor that is switched synchronously with voltage reversal at the two rectifier terminals. The invention also relates to packaged semiconductor devices for such synchronous rectifiers, and to switched mode power supplies (often termed xe2x80x9cSMPSxe2x80x9d) comprising such synchronous rectifiers and/or such devices.
It is known to replace the output diode rectifier (typically a Schottky diode or pn diode) of a SMPS with a low on-resistance field-effect transistor (typically a MOSFET) that is switched in time with the reversal of its terminal voltage. Such a switched-transistor rectifier circuit is known as a synchronous rectifier. Its lower on-resistance reduces the power losses that occurred with the forward voltage drop of a diode rectifier.
Thus, known synchronous rectifiers comprise a field-effect transistor having its source-drain path between the two rectifier terminals, and a gate-control circuit connected to a gate electrode of the transistor for switching the transistor synchronously on and off in accordance with voltage reversal at the two rectifier terminals.
Examples of synchronous rectifiers and their use are disclosed in Section 2.3.3 xe2x80x9cAn Introduction to Synchronous Rectifier Circuits using PowerMOS (Trademark) Transistorsxe2x80x9d, pages 179 to 183 of the Power Semiconductors Applications Handbook 1995 of Philips Semiconductors, published December 1994 (1133011/12000/02/; document order no. 9398 652 85011), and, for example, U.S. Pat. Nos. 6,060,943 and 5,991,182. The whole contents of these three items are hereby incorporated herein as reference material.
As described in these three reference items, the field-effect transistor structure commonly includes a parallel p-n diode that is intrinsically formed by a body-drain p-n junction of the transistor and that is connected between the source and drain of the transistor. This p-n diode is variously known as the xe2x80x9cparasitic body diodexe2x80x9d, the xe2x80x9cintrinsic diodexe2x80x9d, the xe2x80x9cinternal diodexe2x80x9d, and different combinations of these terms. When the transistor is used as a synchronous rectifier, current flow along the source-drain path is opposite to that for normal transistor use, and so the body diode is reverse-biased when the transistor is blocking. The body diode can perform the rectification (but at a much reduced efficiency) if the gate drive of the transistor is lost and/or in the initial start-up state before the transistor is turned on.
Known synchronous rectifiers are not simple replacements for diode rectifiers, because of the need to provide (in addition to the two rectifier terminals) one or more extra terminals for powering the gate-control circuit. The provision of its own power supply rail for this control circuit can be difficult and/or expensive, especially in a SMPS whose output stage is electrically isolated from the input stage. Simple isolated circuits are known using auxiliary transformer windings for driving the transistor, but these also are expensive to implement and poorly controlled so risking excessive voltage on the gate and consequent damage. As a result, the most extensive use of synchronous rectification at the present time is for high-current low-voltage dcxe2x80x94dc conversion circuits where no isolation is required.
It is an aim of the present invention to provide a synchronous rectifier that does not require extra terminals and/or expensive provisions for its powering aspect, and particularly but not exclusively to provide a two-terminal or three-terminal packaged device that can be a simple replacement for two-terminal diode rectifiers.
According to one aspect of the present invention, there is provided a two-terminal self-powered synchronous rectifier circuit wherein the control power for switching the transistor is drawn from the power signal being rectified by the rectifier. Such a synchronous rectifier in accordance with the invention is advantageously formed with a charge pump that is energised by a voltage difference between the two rectifier terminals, the control circuit being powered from the charge pump.
Advantageously a parallel diode is present between the two rectifier terminals to provide rectification during the initial start-up state, until the charge pump is sufficiently energised to power the control circuit for switching the transistor. This diode may simply be the p-n diode intrinsically formed by a body-drain p-n junction of the transistor, but charge-storage effects may then occur if the switching of the transistor is not precisely synchronised, e.g. due to turn-on delay. These charge-storage effects can be avoided by forming the parallel diode as a Schottky diode connected across the body-drain p-n junction of the transistor.
According to another aspect of the invention, there is provided a two-terminal self-powered synchronous rectifier device comprising a two-terminal device package within which the transistor, possible parallel diode, gate-control circuit and its self-generating power supply are accommodated. The self-generating power supply, the parallel diode and the source-drain path of the transistor can be connected between the two terminals of the device package in a parallel configuration.
According to a further aspect of the invention, there is provided a three-terminal synchronous rectifier device comprising a three-terminal device package within which the transistor, possible parallel diode, gate-control circuit and its self-generating power-supply connections are accommodated, the third terminal serving for connection of an external capacitor between the third terminal and one of the two rectifier terminals to complete a charge pump for powering the gate control circuit from the power signal being rectified by the rectifier.
Advantageous technical features of the present invention, and other aspects thereof, are set out in the appended claims.